In a reciprocating internal combustion engine, an air/fuel mixture, introduced into a combustion chamber by opening an intake valve, is compressed by a piston in a cylinder and burnt. The piston reciprocates due to the energy of this burning thereby resulting in motive power. Pressure in the cylinder can make engine starting difficult. However, pressure in the cylinder during operation is desirable to maximize performance and engine efficiency. Therefore, when the revolution of the engine is lower than a given speed, such as during starting, a decompression device can be provided to open the exhaust valve to displace the compressed air/fuel mixture. In particular, auxiliary cam lobes that open the exhaust valve to reduce the pressure of the air/fuel mixture in the combustion chamber before the air/fuel mixture is compressed and burnt can be provided. (For an example, refer to Japanese Patent Publication No. S64-46409.) Decompression devices can include a centrifugal weight which rocks due to the centrifugal force caused by the rotation of a camshaft, and a decompression pin which protrudes from and can be inserted into the cam with the centrifugal weight.
The problem with the prior art systems is that the centrifugal weights and the decompression pins are configured as separated pieces of the decompression device. Accordingly, the overall number of parts of the camshaft system is increased and it is therefore more difficult to combine the parts. Moreover, in the prior art systems, it is necessary to enlarge the cam lobe in order to attach the decompression device thereto.
In view of such a problem, an object of the present invention is to provide a cam mechanism with a decompression device wherein the centrifugal weight and the decompression cam lobe are integrally configured, improving the combining characteristic of the decompression device and achieving a small sized decompression device.